To establish and compare the precision of serum total protein (STP) measured by an optical refractometer to the precision of IgG concentrations measured using radial immunodiffusion (RID), the reference test for quantifying IgG in neonatal calves. 6 sera with previously measured IgG concentration using RID from neonatal beef calves were selected from 3 stratum: low-serum IgG stratum between >5.0 and <15.0g/L(n = 4); moderate-serum IgG stratum between 35.0-45.0g/L(n = 1); high-serum IgG stratum between 60.0-70.0g/L(n = 1). STP was measured 13 times with an optical refractometer. IgG concentrations were measured 28 times with a commercial bovine IgG RID for each sera. The homogeneity of variance within the tests was evaluated with the Levene test (α = 0.10). Unrestricted random sampling bootstrapping (5,000 repetitions) was used to calculate the coefficient of variation (CV) for each serum and test. The homogeneity of variance between simulated test CVs by serum was evaluated (α = 0.10). Differences between simulated test CV by serum were assessed with the Kruskal-Wallis test (α = 0.05). No difference was observed in the variance for STP between sera (P = .39). The average CV for STP was 4.2%, 10.1% for the low IgG stratum, and 15.5% for the moderate/high IgG stratum. Variance differed in serum IgG concentration (P < .0001). Serum with higher IgG concentrations had more variance. Simulated CV for STP and IgG had homogeneity of variance for only 1 sera (P = .31). STP had a smaller CV compared to IgG for every serum (P < .0001). Estimating IgG concentration directly by RID or indirectly by STP lacks the precision that might affect diagnostic interpretation regarding a calf's absorption of maternal antibodies.

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